The subject invention relates to X-ray masks used in X-ray exposure apparatus.
In recent years while semiconductor devices' miniaturization and high integration advanced markedly, vigorous development in minute pattern formation technology have been taking place in the areas of production apparatus and production technology. In the field of developing lithography technology, the optical reduction projection technology, which so far had supported the mass production technology of LSIs, had markedly improved resolution through a shift to ultra-short wavelength light sources and efforts to achieve high NA, is reaching its limits in the half-micron region.
For the 0.25 micrometer level of the next generation exposure technology required for the generations of devices to follow the 256MDRAM, electron beam exposure technology or X-ray exposure technology are considered to be promising.
For X-ray exposure, X-ray masks formed by a combination of X-ray absorbing material and X-ray permeable material are necessary.
A cross-section of a prior art X-ray mask is shown in FIG. 4. In FIG. 4, 1 is the silicon support framework composed of a Si substrate, 2 is the silicon nitride (SiN) film used as the X-ray permeable film (membrane), 3 is the tantalum film used as the X-ray absorbing pattern, 3a is the alignment pattern and 3b is the circuitry pattern.
The circuitry pattern 3b of the X-ray mask 11 is formed on the reverse side of the SiN flim 2, vis-a-vis the X-ray source. Likewise, even in the prior art masks for silicon LSI lithography the circuitry pattern is formed on the reverse side of the mask. This is because the formation of the pattern on the reverse side permits the clean transfer of the mask pattern to the wafer when exposed. And, if the minute pattern were to be formed on the light source side of the mask the pattern would be blurred and not cleanly transferred to the wafer.
Further, since the alignment pattern 3a is formed simultaneously with the circuitry pattern 3b, the alignment pattern 3a is always on the same side of the X-ray permeable film with the circuitry pattern 3b and no thought was given in the prior art to forming the alignment pattern 3a and the circuitry pattern 3b on different sides.
Also, development has been under way on a combination of a silicon nitride film as the X-ray permeable film and a tungsten film as the X-ray absorbant pattern.
X-ray exposure devices, due to reasons such as materials for mirrors and lenses having sufficient capabilities in the soft X-ray wavelength region not having been developed to date (which prevents the realization of reduced projection systems), are using proximity exposure methods which hold the X-ray mask and wafer in parallel with a minute gap between them during exposure.
On the other hand, when forming minute patterns on the 0.25 micrometer level, a high accuracy alignment of 0.1 micrometer or better between the X-ray mask and the wafer is required. For this purpose methods calling for continued high accuracy alignment even during exposure have been adopted.
FIG. 5 shows a typical alignment structure during proximity gap exposure. In FIG. 5, 11 is the X-ray mask shown in FIG. 4, 13 is the highly accurate wafer stage, 14 is the alignment optical system, 15 is the laser beam, 16 is the diffracted beam, 17 is a photodetector, and 18 is the X-rays. The alignment optical system 14 and the photodetector 17 are located outside the exposing X-ray 18's region in order for alignment detection to be possible even during X-ray exposure. After the X-ray mask 11 and wafer 12 are placed oppositely in close proximity, laser beam 15 for detecting alignment is shined over the X-ray mask 11 and alignment marks formed over the wafer 12. The diffracted beam 16, diffracted by the alignment marks, contains the relative alignment slippage data between the X-ray mask 11 and the wafer 12. A highly accurate alignment is achieved by feeding back to the wafer stage 13 the relative alignment slippage data between the X-ray mask 11 and the wafer 12, obtained from the signal detected by the photodetector 17, and correcting the stage position.
We shall explain this in further detail, using FIG. 6.
When aligning the X-ray mask 11 and the semiconductor wafer 12, the method of illuminating with laser beam 15, which is the alignment beam, the grid-shaped alignment marks respectively formed on the X-ray mask 11 and the wafer 12 and comparing each diffracted beam as a method for detecting the amount slippage between the two, is used as one of the most accurate methods (Optical Technology Contact, Vol. 28, No. 7, P. 3 (1990).) This method shall be explained in further detail.
In FIG. 6, the laser beam 15 illuminates X-ray mask 11's gridshaped alignment marks 26, and, of the diffracted beam thus formed, the first order reflected diffracted beam 16 is detected by the photodetector 17. On the other hand, the first order reflected diffracted beam 16a generated by the laser beam which passed through the X-ray mask 11 and illuminated the grid-shaped alignment marks 26a of the wafer 12 are similarly detected by another photodetector 17.
By comparing the diffracted beams 16 and 16a, thus detected, the alignment slippage between the X-ray mask 11 and the semiconductor wafer 12 is detected.
As elements affecting the alignment accuracy, we can cite such items as accuracy in determining the stage position and accuracy of detecting the alignment signal; the alignment signal strength (S/N) in the latter is a vital element.
In accordance with the above prior art structure, in detecting the alignment signal by the photodetector 17, as shown in FIG. 7, the laser beam 15 makes a round trip through the X-ray permeable film having a permeability ratio of around 70% formed by the silicon nitride film 2 of the X-ray mask 11, the strength of the diffracted beam 16 is reduced to around 50%. Moreover, the permeability ratio of the SiN film varies cyclically with its thickness, and, depending on the conditions, it is possible that the strength of the diffracted beam 16 will be decreased further.
Also, the diffraction efficiency of the alignment marks in this case is not dependent on the marks' shape but on the effective refraction ratio determined by the combination of the materials used in the X-ray permeable film and the X-ray absorbing pattern forming the X-ray mask 11, and usually becomes a small value so that it is not possible to freely set the optimum value.
Due to these reasons, the alignment signal strength became 1/5 or less of the ideal signal strength and had been a major obstacle to high accuracy alignment.